1. Field of the Invention
The present invention relates generally to the attenuation of electromagnetic energy and, more specifically, to porous materials incorporating electromagnetic-energy-attenuating materials.
2. Description of the Prior Art
As used herein, the term EMI should be considered to refer generally to both electromagnetic interference and radio frequency interference (RFI) emissions, and the term “electromagnetic” should be considered to refer generally to electromagnetic and radio frequency.
During normal operation, electronic equipment typically generates undesirable electromagnetic energy that can interfere with the operation of proximately located electronic equipment due to EMI transmission by radiation and conduction. The electromagnetic energy can exist over a wide range of wavelengths and frequencies. To minimize problems associated with EMI, sources of undesirable electromagnetic energy can be shielded and electrically grounded to reduce emissions into the surrounding environment. Alternatively, or additionally, susceptors of EMI can be similarly shielded and electrically grounded to protect them from EMI within the surrounding environment. Accordingly, shielding is designed to prevent both ingress and egress of electromagnetic energy relative to a barrier, a housing, or other enclosure in which the electronic equipment is disposed.
In the abstract, an ideal EMI shield would consist of a completely enclosed housing constructed of an infinitely conductive-material without apertures, seams, gaps, or vents. Practical applications, however, result in an enclosure constructed of a finitely conducting material having some apertures. Apertures may be unintentional, such as those incident to a method of construction (for example, gaps or seams, for example between adjacent access panels and around doors, or between component housings and circuit boards), or intentional, such as vents to accommodate air flow for cooling. Special methods of manufacture may be employed to improve the shielding effectiveness of unintentional apertures, for example, by welding or soldering seams, or by milling a cavity within a contiguous member of shielding material, thereby eliminating unintentional apertures.
As mentioned, cooling vents are typically required because electronic equipment typically generates thermal energy (that is, heat) that must usually be removed from the equipment to ensure continued, long-term, and proper operation. Shielding of apertures relating to cooling vents are necessarily more challenging, because the apertures themselves cannot be eliminated as cooling air must be allowed to pass through to facilitate heat transfer.
Prior-art solutions are available that provide some level of EMI shielding across a cooling aperture. For example, a cooling aperture may be covered by an electrically conducting plate having field of smaller apertures (that is, a two-dimensional array) spanning the cooling aperture. Other solutions include an electrically conductive screen, while still other solutions include a two-dimensional array of waveguide apertures (for example, a “honeycomb”). Each of these solutions provides preferential attenuation to lower-frequency EMI having a frequency below some “cutoff” frequency generally determined by the largest dimension of each individual aperture. Moreover, these solutions are complicated as they rely on a positive electrical bonding of the plate or screen to the equipment housing that must be maintained over the life of the equipment. Maintaining such an electrical bond can be particularly challenging in high-vibration and/or corrosive environments.
As mentioned, shielding effectiveness of such conventional methods and materials decreases with increasing frequency. Thus, effective shielding of EMI in many of today's electronic applications is becoming more challenging, as current trends continue to increase operational frequencies. For example, microprocessor clocking rates used within currently available consumer electronics, such as personal computers, are operating at thousands of megahertz. Later generation devices are expected to operate at even greater frequencies.
There exist other methods for providing EMI shielding across cooling apertures. See, for example, U.S. Pat. No. 5,151,222 issued to Ruffoni, the disclosure of which is herein incorporated by reference in its entirety. Ruffoni discloses the use of an open-cell reticulated polyurethane foam impregnated with a conductive ink. The method disclosed in Ruffoni applies the conductive ink to the surface of the foam, resulting in a variation, or gradient, in conductivity from the coated surface of the foam to its interior. Purportedly, the conductive ink offers improved attenuation performance at higher frequencies. Unfortunately, however, due to the resulting gradient, a heavier application of conductive ink is required at the foam surface in order to provide a desired overall attenuation characteristic. Such an application results in a pressure drop from blocked pores due to a heavier application of conductive ink necessary to meet increasing attenuation requirements. As such, the foam shown in Ruffoni is not suitable for use as an air filter.